Letter to the Editor

HIFI spectroscopy of low-level water transitions in M 82 ^*

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: aweiss@mpifr-bonn.mpg.de
² Observatorio Astronomico Nacional (OAN) – Observatorio de Madrid, Alfonso XII 3, 28014 Madrid, Spain
³ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ IRAM, Avenida Divina Pastora 7, 18012 Granada, Spain
⁶ NASA Science Center, Caltech, Pasadena, CA, USA
⁷ Centro de Astrobiologia (INTA-CSIC), Ctra de Torrejon a Ajalvir, km 4, 28850 Torrejon de Ardoz, Madrid, Spain
⁸ KOSMA, I. Physikalisches Institut der Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
⁹ N. Copernicus Astronomical Center, Rabiańska 8, 87-100 Toruń, Poland
¹⁰ Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 91109, USA
¹¹ Experimental Physics Dept., National University of Ireland Maynooth, Co. Kildare, Ireland
¹² European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
¹³ Atacama Large Millimeter/Submillimeter Array, Joint ALMA Office, Santiago, Chile

Received: 28 May 2010
Accepted: 1 July 2010

Abstract

We present observations of the rotational ortho-water ground transition, the two lowest para-water transitions, and the ground transition of ionised ortho-water in the archetypal starburst galaxy M 82, performed with the HIFI instrument on the Herschel Space Observatory. These observations are the first detections of the para-H₂O(1₁₁–0₀₀) (1113 GHz) and ortho-H₂O⁺(1₁₁–0₀₀) (1115 GHz) lines in an extragalactic source. All three water lines show different spectral line profiles, underlining the need for high spectral resolution in interpreting line formation processes. Using the line shape of the para-H₂O(1₁₁–0₀₀) and ortho-H₂O⁺(1₁₁–0₀₀) absorption profile in conjunction with high spatial resolution CO observations, we show that the (ionised) water absorption arises from a ~2000 pc² region within the HIFI beam located about ~50 pc east of the dynamical centre of the galaxy. This region does not coincide with any of the known line emission peaks that have been identified in other molecular tracers, with the exception of HCO. Our data suggest that water and ionised water within this region have high (up to 75%) area-covering factors of the underlying continuum. This indicates that water is not associated with small, dense cores within the ISM of M 82 but arises from a more widespread diffuse gas component.